Ink jet printers form images on paper by ejecting ink droplets from an array of nozzles on an ink jet print head. In thermal ink jet print heads, a heating element, such as a resistor, is associated with each nozzle. The heating element heats adjacent, thereby causing formation of a rapidly expanding bubble of ink. The expanding bubble causes a droplet of ink to be ejected from the nozzle.
Generally, each heating element is activated by a corresponding switching device, such as a MOSFET drive transistor, that is connected electrically in series with the heating element. Since these transistors must handle relatively high current levels, they also generate heat that is directly related to their on-resistance. The heat generated by the drive transistors can significantly affect the temperature of the print head chip, and can cause temperature gradients across the chip. Variations in print head temperature cause variations in ink droplet mass, which, in turn, degrade print quality. Therefore, it is desirable to keep the on-resistance of the drive transistors as low as possible.
As the state of the art advances, the spacing between nozzles in ink jet print heads decreases, thus allowing higher print resolution. As nozzle density increases, so does the density of heating elements and drive transistors associated with the nozzles. As the width of drive transistors decreases to accommodate high-density packaging, maintaining low on-resistance becomes much more challenging.
Therefore, a drive transistor that accommodates high-density packaging while maintaining low on-resistance and high-current carrying capability is needed.